Secularism is a Christian phenomenon. It is now common for secularists to shy away from the term 'atheism,' their contention being that it is an identity based on a negation. What many of them don't fully grasp is just how little the term actually negates. If it is a-theos, it is certainly not a-moral (many secularists will be the first to tell you). But most of the time, it doesn't even negate Christian morality--only the means of arriving there have been denied.
Christianity teaches that Jesus came to fulfill The Law. The paradox of this statement is that Christians were largely distinguished by their disregard for Levitical Law (at least, many sects and factions). They believed not in circumcision of the body, but circumcision of the heart. Hate was as bad as murder and lust was as bad as adultery. In other words, what had once been observed by physical law was raised to a spiritual standard. The law was turned inward.
The secular inheritance of Christianity goes something like this: God, the Law, embodied himself in a man, who turned the law inward. The man died, rose, ascended, and left us with the holy spirit; not a law, but a reminder. That reminder is what secularists kept, so to speak.
Many theological precepts pepper secularist vocabulary. They emphasize Truth (we keep it in the back of our minds that it will make us free). I've heard many secularists say that they practice Christian morals better than most Christians do, because they can radicalize the message without letting dogma or biblical literalism getting in the way. This isn't much different from Jesus turning Levitical Law inward.
I didn't bring this to light to spiritualize secularism or to secularize Christianity, but to show that, perhaps if they have common features, then surely they have common goals. If they help one another, they can learn from one another (either one might seek to convert the other but I'll leave that to them). Both systems of thought have more in common today than they have in difference.